Alkali activation of fly ash with proper mix design and correct
formulations can exhibit very good strength and chemical resistance
and other potentially valuable characteristics similar or even better
than conventional Portland cement. Various geo polymeric products
is not difficult to achieve on a laboratory scale by optimization
of process conditions, the ability to understand the flow ability,
repeatability using variable material sources, control the setting
process have always present to be issues in large scale production. A
study was conducted on leaching of fly ash hybrid activator solution
namely NaOH, Na2SiO3, for preparing geo polymer. Geo Polymer (GP)
concrete was made with the addition of organic admixture, Sodium
naphthalene formaldehyde sulphonate, Cetyl Tetra Ammonium
Bromide, Modified polycarboxylate super plasticizers by varying
the dosage from 0-2% by keeping the Liquid/Solid as 0.45. The
specimens were cured for 70 C for 24 hours. Flow ability for concrete
were studied. Microstructure of the mortar was studied by FTIR and
SEM/EDAX. The Compressive strength of the mortar in the order 30
Mpa was achieved with good flow property by the addition Modified
Poly Carboxylate (MPC) and the zeta potential value was found to be
similar in the order of control specimen(-17.4mv)

World's Ordinary Portland cement industry production
increases with the increasing demand of building industry which
is crossing a million tons per year. At the same time fly ash, an
industrial byproducts from coal based thermal power plant,
which is available abundantly are underutilized worldwide consumption is about 20–30% of the generated fly ashin
cementious products, construction areas such as highway road
bases, grout mixes, stabilizing clay based building materials [1-5].
It has tremendous potential applications of new areas in recent
years [6]. One of the best ways is to synthesize, geo polymer a
binder made by alkali-activated solution because of its low-cost,
environment-friendly, energy-saving and resource-recycling
benefits.

The key processes involved in the synthesis of geo polymers
are: (a) Dissolution of fly ash or similar alumino-silicate
precursors to provide the Al and all or part of the Si constituents
needed. (b) hydrolysis reaction to generate aluminum and
silicate species and finally (c) condensation of these species
and/or silicates from the activator to build up geo polymeric
network structures. Geo Polymers (GP) with frameworks of
[SiO4]4−and [AlO4]5− tetrahedral linked alternately by sharing all
the oxygen atoms, are generally amorphous alumino silicates
or semi-crystalline zeolites depending on the alkaline activator
used in their manufactures and reaction conditions. The negative
charge of [AlO4]5− tetrahedron on skeleton is balanced by extraframework
cations of Na+, K+ and Ca2+ ions.[7] Corresponding to
different Si/Al ratios, the geo polymers are composed of network
structures of polysialate (-O –Si- O- Al -O), polysialate siloxo (-OSi-
O -Al –O- Si –O-), and polysialate disiloxo (- O –Si- O- Al- O
–Si- O –Si- O).

The compressive strength and the workability of geopolymer
concrete are influenced by the proportions and properties of
the constituent materials that make the geopolymer paste. The
available literature results have shown the following: [5, 8-11]

The interaction of various process parameters, nature of
admixtures on the compressive strength and workability of Geo
polymer products are complex and hence final Geo polymer
products exhibit different performance properties viz. high
compressive strength, high thermal resistance, low shrinkage&
low thermal conductivity, high level of resistance to a range of
different acids and salt solutions etc. It is also important to note
that all Geo polymer products will result all of these properties
or in other words, no single formulation that can optimize all the
properties. Hence formulations recipes are required to tailor
made Geo polymer mixes to attain the required specifications and
technical performance. This will be possible only with thorough
understanding of the raw material reactivity, chemistry, role of
admixtures and reaction conditions.

The chemical admixture especially, high range water reducing
admixtures commonly known as Super plasticizers have become
essential component in ordinary Portland cement concrete. The
commercial super plasticizers viz., sulphonate lignin, Sulphonate
Naphthalene Formaldehyde (SNF), Sulphonate Melamine
Formaldehyde (SMF), and poly carboxylates which are used in
the production of Portland cement Concrete where dispersion of
cement particles by electrostatic repulsion through adsorption
process enhance ethe workability, giving high performance
and improve durability. The mechanism and working nature of
super plasticizers in OPC have been well reported in literature
[12-14]. Such type of chemical admixtures does not work in Geo
polymer system because of high alkaline conditions. [15] The effect
of third generation SP such as vinyl co polymers, poly acrylate
co polymers along with modified form of melamine, naphthalene,
polycarboxylate ester based on alkali activated slag and fly ash
based have been reported [16-18]. It was found that addition of
N-based SP improved the workability up to 2% mass of binder
without affecting the strength. Studies on evaluation of PC based
SP on fly ash Geo polymer showed appreciable reduction in the
strength around 20 to 35% with reference to original concrete.
To ensure uptake of Geo polymer production at larger scale in
the construction industry, research is needed in the critical area
of suitable admixture for improving workability and durability.

Hence the present study, Physio chemical characteristics
of raw materials used for geo polymerization reactions,
microstructure of Geo polymer products, effect of water reducing
admixtures on performance have been studied.

Materials and Methods

Fly Ash
Class F Fly Ash sample collected from Thermal Power Plant,
Gummidipoondi, India, used as a Geo Polymer Source Material
(GSM) The chemical composition of the low lime fly ash is
determined by EDXRF analysis (Bruker) and is summarized in
Table 1.

Activator
Sodium silicate solution was used as an alkali activator
solution. (15% Na2O, 33% SiO2 and 52% H2O) The starting sodium
silicate solution modulus was 2.0 and it was further adjusted by
adding lye (50% NaOH).

Preparation of Alkali Activator Solution:
The commercially available 50% sodium hydroxide
solution (i.e., Caustic lye) was mixed with distilled water in a
closed container, the temperature measured by using Infrared
thermometer was about 42ᴼC. Therefore, prepared sodium
hydroxide solution was kept for 24 hrs in order to bring down
to room temperature and mixed with Sodium Silicate Solution of
molar ratio 2.0, then stirred well for homogeneous mixing.

Mixing, Casting and Curing:
The materials were weighed accurately using digital
electronic weighing balance. Sand and Fly Ash (Geo polymeric
source material, GSM) were dry mixed for about 3 minutes in
a digital mortar mixer till a uniform mix is achieved. Then, the
AAS was poured into it and the mixing continued for further 7
minutes. SP's dosage were varied from 0 .5% ,1%, 1.5%,2.0% by
mass of fly ash and the whole mix was blended for 4 min with the
speed of 300 rev/min. The mix proportions of mortar specimen
are given in Table 3. The fresh mix was then placed in to plastic
cylindrical moulds of size 50 mm diameter x 100 mm height in
three equal layers and compacted using a laboratory vibrator.
After 24 hrs of casting, the cylindrical moulds, containing the geo
polymer mortar matrix, were kept for heat curing at 60ᴼC for a
period of 24 hrs in a hot air oven. Then specimens were allowed
to cool to room temperature before demoulding operations.

Chemical composition of low lime fly ash is presented in Table.1

Component

(wt. %)

SiO2

Al2O3

CaO

Fe2O3

K2 O

MgO

Na2O

FA

47.55

33.45

2.099

10.17

1.65

0.005

0.015

Properties of SP's are shown in Table .2

Superplasticizers

Chemical base

Appearance

pH

Specific gravity

Solid content

SNF

Sodium naphthalene formaldehyde sulphonate

Dark brown liquid

7

1.2-1.5

40-42%

CTAB

Cetyl Tetra Ammonium Bromide

White powder

5-7

1.5

38-40%

MPC

Modified polycarboxylate

Colorless liquid

3-4

1.4

42-45%

Details of mixture proportions are presented in Table 3:

GP mix design

Mortar mix design

Activator solution

Fly ash

(By Weight)

Sand

(By Weight)

AAS/FA

(By Weight)

Na2O/Al2O3

H2O/Na2O

Si2O/Al2O3

FAGP

0.5

1

0.5

0.43

3.35

0.5

Test methodology

X-Ray characterization studies using X Phillips pW 1710"
(Cu Kα = 1.54178) and XRD pattern is shown in Fig 1. Micro
structural characterization has been done by Scanning Electron
Microscopy (Bruker) and FT-IR by KBr pellet technique (Perkin
Elmer). The workability of fresh geo polymer concrete mixtures
was tested by Slump test following ASTM: C1437. Flow of fresh
geo polymer mortars was measured in accordance with ASTM
C1437-07 to determine workability loss of fresh concretes. The
consistency of the mixes was measured using a Vicat needle as
described in ASTM C191-08. Malvern Zetasizer (Nano series)
was used to measure the zeta potential with 0.5 weight % of
Geo polymer matrices in deionized water against the standard
potassium tungsto -silicate solution and calculations were made
through Zetasizer software. The compressive strength test was
performed on the specimens on 3, 7 and 28 days, with a loading
rate of 0.33 MPa/s in a Controls CTM machine. Average of 3 test
results was taken for calculations.

Results and discussion

Mineralogical studies of source material

X-ray diffraction pattern analysis of fly ash contain mineral
phases like quartz (PDF#46-1045), magnetite, mullite (PDF#85-
1456), anorthite, hematite (PDF#88-2359), and other minerals.
Amorphous content in the fly ash has notbeen quantified from
XRD. The glassy phase content of fly ash was determined by
dissolving 1 g of it in 100 ml of HF(1%) acid –(Arjunan's method)
with constant stirring for 6 hrs [19]. Dried samples (110°C)
were weighed and the content of glassy phase was determined
by weight loss. HF dissolve the glassy phase of fly ash, while
crystalline phases (usually qurartz, mullite, haematite and
magnetite) remains intact. In our study, the total quantity of SiO2
was found to be 48% wherein amorphous content was 35%.
Which are essentially required for the initiation step of the geo
polymerization process, yielding higher amounts of reactive SiO2
and Al2O3 and further to combine during the agglomerate phase.

FTIR Spectrum of source material

Fig.2 depicts the FTIR spectrum of FA, activated fly ash with
(FAGP1) and without addition of SP (FAGP). The original Fly ash
precursor contains both 'active' and 'inactive' bonds indicating their reactivity in the alkaline solution. The active sharp
absorption band at around 1093 cm-1 (s) related to asymmetric
stretching of (Si, Al IV)-O-Si in glass and partially overlapped by
phases of mullite and quartz. The vibrational frequency at 990
cm-1(m) which corresponds to the asymmetric stretching of (Si, Al
IV)-O-Si in amorphous glasses which could be composed of higher
Al concentration. The vibrational frequency at 793 cm-1(w) and
555 cm-1(w) was assigned to the symmetric stretching of Si-O-Si
in quartz and AlVI symmetric stretching of Al-O-Si in Mullite or
Mullite like structure. These bonds are said to be 'inactive'. FTIR
spectrum of activated fly ash (FAGP) exhibiting the broad peak
at around 1000-1100 cm-1(b) are attributed to T-O asymmetric
stretching vibrations and represent the fusion of both Al-O and
Si-O symmetric stretching. The lower wave numbers are related
to a larger extent of aluminium incorporation into the silicate
backbone. The broad bands at about 3453 cms-1(b) and1643 cm-

Figure 1: XRD pattern for fly ash (Indian).

Figure 2: FT-IR spectrum of Fly ash and fly ash geopolymer.

1(m) correspond to stretching vibrational frequency and bending
vibrational frequency of adsorbed water molecule formed during
the reaction. Also it has an Si-O-Al stretching vibration at 775
cm-1 and Si-O-Al bending vibration at 433 cm-1[19]. Therefore, the
original silicate and or/alumina-silicate structure in raw material
have been significantly depolymerized through leaching during
alkali activation. The FTIR of FAGP1 spectra, the additional peaks
around 2924 cm-1 reflected the presence of aliphatic –CH bond
and doublet at around 2360 cm-1 correspond to -NH2 group which
are due to the organic amino admixture added in the specimen.

Effect of addition of SP on workability of activated fly
ash concrete

The slumps of the activated fly ash concrete with the addition
of different SP are shown in the Fig.3. The increase in relative
slump was found to be 40%, 35% for the concretes added with
MPC and SNF respectively whereas for CTAB additions do
not show any improvement in slump. This could be due to the
instability of the compound at high alkaline medium (pH level of
AAS is around 13) [18, 20]. Generally, most of the commercially
available SP to OPC was found to be not suitable for alkaline
activator of fly ash. In the case of MPC, existence of several lateral
chains in its structure results in steric repulsion that compensates
the tendency of particles to form complexes, therefore their
plasticizing ability would be greater than SNF based SPs.

Figure 3: Slump of fly ash based geopolymer with addition of SP.

Figure 4: Compressive strength of Fly ash GP (a) 7 days (b) 28 days.

Compressive strength

The compressive strength of mortar mixes with and without
addition under heat curing conditions for different curing days
have been carried out. Control specimens, FAGP showed gradual
increase from 5 MPa (3d), 17Mpa (7 d) to 30 MPa at 28days.
The compressive strength of the mortar at 7 days and 28 days
with and without using different SPs is shown in Fig. 4(a) & 4(b).
Generally, additions of SP in cement matrix do not affect the
strength significantly. In present study, the compressive strength
of the mortar by using all types of SPs (i.e. SNF, CTAB and MPC
based SPs) was decreased with respect to that of the control
mortar. The decrease in compressive strength was 57%, 48%
and 10% for the mortars with SNF, CTAB and MPC respectively.
The SNF and CTAB have shown the reduction in compressive
strength may be due to the instability of these SPs in a very
high alkaline medium (pH level of AAS is around 13) but MPC
based SP's has shown higher plasticizing (workability) and less
negative effect on the compressive strength compared to N and
M based SPs [20].

Zeta potential (ξ)

Zeta potential is a scientific term for electrokinetic potential
[21]. It is a measure of the magnitude of the electrostatic or
charge repulsion/attraction between particles, and is one of the
fundamental parameters known to affect stability.

Fly ash contains a reactive silicate and aluminate groups
on its surface it has negative zeta potentials. During the
geo polymerization reaction, activator solution having OHions
react with the aluminate species on the surface of fly
ash forming[Al(OH)4]- and with silicate species form either
[SiO(OH)3]-or [SiO2(OH)2]2-, thereby establishment of negatively
charged double layer. Further Na+ ions of the alkaline activator
react to form a sodium alumino silicate gel layer [22]. There by
smaller negative values will be expected.

In the case of FAGP, the small zeta potential (-15.4 mv) was
observed, this is due to the accumulation of more Na+ ions in
the double layer while forming gel resulted in a decrease of zeta
potential and higher compressive strength. The addition of MPC
in FAGP (-17.6 mv) shows small change in zeta potential and It is
also reflected in compressive strength.

With the addition of SNF and CTAB in FAGP leads to
more negative zeta potential such as -44.6 mv and -35.5 mv
respectively. The agglomerated fly ash particles are found to be
destabilizes by the addition of CTAB and SNF as seen from the
more negative values, which also evident from their compressive
strength [23].

Morphology

The morphology of fly ash precursors observed from
SEM photograph showed that fly ash particles are appeared
to be spherical as thin walled hollow sphere with the size of 1
micron Fig 6(a). SEM picture of alkali activated fly ash mortar
after 28 days of curing time Fig.6 (b) clearly showed that dense
gel formation over the surface of fly ash with fewer unreacted
particles might be the presence of mullite or quartz phase.

Figure 5: Zeta potential of fly ash and geopolymer suspensions in water

Figure 6: SEM image of (a) original fly ash (b) FAGP.

Figure 7: EDAX of FAGP.

Conclusion

Fly ash activated by hybrid mixture of sodium hydroxide
and sodium silicate solution was found to be appropriate for
the geopolymerisation reaction with acceptable compressive
strength (30 MPa).The adsorption of organic admixture in the
alkali leaching fly ash reaction might contribute the improvement
of flow properties of the precursors evident from the slump test
to the concrete mix. Further to note MPC based SP to the GP
concrete mix restore the compressive strength at the satisfactory
level.The microstructure of geopolymer formed in our mix
design studied by SEM/EDAX revealed that the compact dense
microstructure of the matrix with lesser number of unreacted
fly ash particles. The FTIR spectra showed the prominent Si-O-Si
vibartion(990 cm-1) indicate the degree the geopolymerisation.
The results from EDAX indicate that major components in GP
paste were silicon and aluminum having Si/Al ratio 1.5. with
small amount of sodium.

Acknowledgement

We gratefully acknowledge financial support from the SRM
University and Department of Science and Technology (DST)
under grant no DST/TSG/2012/20.